Since thermoplastic polyurethanes based on polyesters and aromatic polyisocyanates will ultimately discolor, only mild interest has been displayed in the past in providing products with low initial color.
Recently, there has developed a market for colorless, transparent roller skate wheels molded from thermoplastic polyurethanes. The specific type of discoloration of interest was that which occurred during exposure of the thermoplastic polyurethane to processing temperatures experienced during drying, extrusion, or molding. The yellowing of the polymer did not seem to be associated with any significant change in strength properties and was entirely different from the discoloration which occurs during the service life of the ultimate molded or final product (i.e., weathering). It was found that this discoloration could occur during pelletization extrusion resulting in the shipment of pellets to the molder which varied in color. In fact, it was found that some lots of thermoplastic polyurethane discolored even more during the maufacturer's drying and/or molding operations with the result that the ultimate product also varied in color.
Finally, although many thermoplastic polyurethanes exhibit this tendency to discolor to varying degrees, the problem is most pronounced in those polyurethanes prepared from aromatic isocyanates and polyesters wherein the only diol used is 1,4-butanediol.